Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 MWh		Dec. 31, 2012 MWh
Credit Concentration Risk [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	10
Credit Concentration Risk [Member] | Investment Grade Ratings from Standard & Poor's, Moody's, or Fitch Ratings [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	28.6
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	24.00%
Credit Concentration Risk [Member] | No Investment Grade Ratings from External Credit Rating Agencies [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	6
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	65.7
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	56.00%
Interest Rate Swap [Member]
Interest Rate Derivatives [Abstract]
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months	0.5
Electric Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	570,000	[1],[2]	813,000	[1],[2]
Natural Gas Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	31,000	[1],[2]	646,000	[1],[2]
Vehicle Fuel Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	284,000	[1],[2]	307,000	[1],[2]

[1] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

[2] Amounts are not reflective of net positions in the underlying commodities.